1. Field of the Invention
The present invention relates to a secondary battery. The present invention also relates to a solar power generating system, a wind power generation system, and a vehicle provided with a secondary battery. The present invention further relates to a method for fabrication of a secondary battery.
2. Description of Related Art
In recent years, as consumer electronics, such as cellular phones, portable electronic appliances, personal digital assistants, are rapidly made compact, lightweight, and versatile, for use as their electric power sources there has been strong demand for the development of secondary batteries that are compact and lightweight, that offer high energy density, and that stand repeated charge-discharge cycles for a long period of time. As secondary batteries that meet those requirements, most promising are lithium-ion secondary batteries, which offer higher energy density than other secondary batteries, and a wide range of research is being conducted to develop lithium-ion secondary batteries with increasingly enhanced properties.
Moreover, in recent years, in view of environmental issues such as global warming, lithium-ion secondary batteries have been increasingly used for storage of electric power in solar power generation systems, wind power generation systems, and the like. Furthermore, as measures to reduce CO2 emissions and cope with energy issues, there has been a high expectation for the spread of fuel-efficient, low-emission vehicles, such as hybrid electric vehicles (HEVs) and electric vehicles (EVs), and the development and commercialization of lithium-ion secondary batteries intended for use as vehicle-mounted batteries are underway.
As discussed above, the application of lithium-ion secondary batteries is no longer limited to portable appliances such as cellular phones but is now widening into the driving of large motors as in electric vehicles. As the demand for lithium-ion secondary batteries increases, they have come to be expected to offer high capacities and long lives of 500 cycles or more.
On the other hand, it is known that the capacity of a secondary battery whose capacity has diminished can be restored by refilling it with fresh electrolyte liquid. For example, JP-A-2001-210309 proposes a secondary battery provided with an refill tap which is removable to allow refilling of electrolyte liquid.
Inconveniently, however, with the secondary battery disclosed in JP-A-2001-210309, the battery capacity is restored by refilling of electrolyte liquid, but it is not that the life until the refilling is improved. Thus, each time the battery capacity diminishes, the battery needs to be refilled. Moreover, the refilling of electrolyte liquid needs to be conducted in a low-humidity environment, and thus requires special equipment such as a humidity-controlled glove box. This makes the refilling difficult, and thus makes it difficult to improve the life of the battery. In particular, a secondary battery used for storage of electric power or the like is bulky and troublesome to move, and this makes extremely difficult its maintenance such as refilling. Thus, with the conventional secondary battery disclosed in JP-A-2001-210309, it is practically impossible to improve the life of the battery.